It's not 'breaking the sound barrier'. B2's are not supersonic aircraft. It can do about Mach 0.95 flat out although its usual cruising speed is about the same as a 747. This is 'transsonic', not supersonic.

You can get a Prandtl-Glauert singularity at subsonic velocities under the right conditions (high humidity, high g or large angle of attack).